As polyolefin articles are fabricated in melt processing, the polymer crystallizes. Crystals tends to organize into clusters. DBS-based compounds enhance nucleation and clarification of polyolefins by providing a fibrous network in the polymer. DBS derivatives provide a fibrous network that serves as a template for polyolefin recrystallization that results in faster and more orderly crystal growth during the polymer cooling process.
DBS derivative compounds are usually prepared by condensation of two moles of an aromatic aldehyde with one mole of a polyhydric alcohol such as sorbitol or xylitol. Examples of this process can be found in Murai et al., U.S. Pat. No. 3,721,682; Murai et al., U.S. Pat. No. 4,429,140; Machell, U.S. Pat. No. 4,562,265; Kobayashi et al., U.S. Pat. No. 4,902,807; and Scrivens et al., U.S. Pat. No. 5,731,474. DBS and its derivative compound bis-(3,4-dimethyl benzylidene) sorbitol (3,4-DMDBS, or simply “DMDBS”) and polyolefin articles containing these compounds also have been disclosed in U.S. Pat. Nos. 6,586,007 and 5,049,605. These compounds are known as additives that impart relatively low haze.
Haze is a measurement of the amount of transmitted light that is scattered as the light passes through an article. Haze is expressed as a percentage (ASTM Method D1003-00). The greater the haze of an injection molded polypropylene article, the more opaque the article appears. Low haze is highly desirable not only from an aesthetic perspective, but also in many applications from a utilitarian perspective. For example, in consumer applications, it is desirable to clearly see the contents of a food storage container without having to remove the lid. It may be crucial to clearly see the volume in a syringe being used for delivery of medications, in medical plastics applications. Thus, low haze is very desirable for many applications.
Typical haze values for articles containing the above referenced individual compounds are roughly 8% for 3,4-DMDBS and about 20% for unsubstititued DBS (called simply “DBS”). These quoted haze values were taken in random polypropylene copolymers (RCP) when injection molded at about 230° C. in 50 mil plaques containing about 2000 ppm of total clarifier loading.
Melt flow index (MFI), also known as melt flow rate (MFR), is a measure of viscosity of a material (such as a polyolefin resin) at a given temperature, expressed as grams/10 minutes. ASTM 1238-04 is an Internationally known standard for determining the rate of extrusion of molten resins through a die of specified length and diameter under prescribed conditions. ASTM D1238-04 (Procedure C) is directed to automatically timed flow rate measurements for high flow rate polyolefins using half height, half diameter dies. In general, the lower the viscosity of a material at a given temperature, the higher will be the MFR of that material.
There is a continuing demand in the industry for improved polyolefin processes and products. In manufacturing of such articles, cost of manufacture is greatly influenced by cycle time. That is, cycle time is the time interval for a complete molding cycle, i.e. the time required to close the mold, inject liquid plastic, mold the part, and eject the part. Typically, cycle time may be influenced negatively when a higher processing temperature is required, which results in prolonged cooling time. Furthermore, the cost in energy consumption would be higher using higher processing temperatures. Thus, short cycle times and lower processing temperatures are desirable. However, the limits required in terms of cycle time and processing temperature are dictated by the resin, the processing conditions, and the behavior of the nucleating agent(s) in the resin. These factors may be very unpredictable.
In the use of DBS derivatives, it is known that bis(3,4-dimethylbenzylidene) sorbitol (known commercially as Millad® 3988, or “DMDBS”; sold by Milliken & Company) is an excellent nucleating agent, and is capable of contributing to relatively low haze levels in polyolefins. It is also known, however, that this compound at very low processing temperatures may exhibit undesirably high levels of haze.
U.S. Pat. No. 6,586,007 to Lake et al (assigned to Milliken & Company, a common assignee to the present application) discloses in Table 1, comparative examples 16 and 17 the use of a blend of bis(3,4-dimethylbenzylidene) sorbitol (3,4-DMDBS) and dibenzylidene sorbitol (DBS). This blend is disclosed for use in a base resin processed at about 230 degrees C. through four heating zones, and then subjected to a molding temperature of 220 degrees C. See U.S. Pat. No. 6,586,007; column 1, lines 60-67. Results reported in the patent are shown graphically in FIG. 1 of the present application, as further discussed herein.
Japanese Patent Application Hei 6[1994]-93764 published Oct. 24, 1995 discusses a method of synthesizing the following species A, B, and C together in a single reaction system: Component A (asymetrical): 1,3-o-3,4-dimethylbenzylidene-2,4-o-benzylidene sorbitol and/or 1,3-o-benzylidene-2,4-0-3,4-dimethylbenzylidene sorbitol; Component B (symmetrical): bis(o-3,4-dimethylbenzylidene) sorbitol; and Component C (symmetrical): dibenzylidene sorbitol.
It would be highly desirable to develop a composition, process, or polymeric article manufacturing system that avoids many of these difficulties. There is a need to facilitate manufacture of low haze articles with reduced amounts of heat energy, and with reduced molding cycle times. A composition that is useful for application to high MFR resins also would be desirable, as high MFR resins are less viscous, and may be processed at lower temperatures. A product, composition, or process that provides increased article manufacturing output is desirable. It would be desirable to find a nucleating agent composition or mixture that provides a low degree of haze (i.e. high transparency) in a finished polymeric article, with less energy consumption and higher production efficiency.